French Patent Application No 2609046 teaches hydrolysis of starch in the presence of starch hyrdrolyzing enzymes. Starch is known to be extracted from food and grain plants. One in the art knows that starch is easily hydrolyzed due to its chemical bonds, whereas cellulose is not easily hydrolyzed. The patent teaches to add water to the ground dry starch to adjust the concentration in ground starchy substratum, expressed in dry matters, to a value understood between 50 and 400 g/liter of the middle of fermentation. The French patent application mentions nothing of applicability to cellulosic feedstocks.
It is known in the art that the energy cost of distillation of ethanol from a fermentation process is dramatically reduced if the fermentation broth contains more than 4% ethanol. This requires a sugar concentration above 8% (w/w), which with most types of biomass, in particular lignocellulosic biomasses, corresponds to an initial dry matter content above 20%.
It is therefore critical to be able to utilize lignocellulosic-containing biomasses with high dry matter contents, preferably above 20% by weight.
Enzymatic hydrolysis of biomass has previously been described. However, in the case of lignocellulosic biomasses, only material consisting in fibers and particles with an average size below linch (25.4 mm) and furthermore having a relatively low dry matter content, i.e. below 20% (w/w), have successfully been hydrolyzed.
Enzymatic hydrolysis of biomass has traditionally been carried out in stirred tank reactors equipped with impellers (e.g. Rushton turbine or Intemig impeller) mounted on a centrally placed impeller shaft similar to what is used in the fermentation industry. Due to this equipment, high viscosity solutions, very sticky or very dry material cannot be stirred efficiently and will result inhomogeneous, maintaining areas with very poor or no mixing. Furthermore, stirring of such solutions requires very large energy inputs, which are detrimental to the economy of the process. Operating with polysaccharide-containing biomasses has therefore previously restricted the upper possible limit to app. 20% (w/w).
This is evidenced in U.S. Pat. No. 4,409,329 which describes hydrolysis of solid cellulose material to sugar, where cellulose is hydrolyzed to simple sugars by treating a granular slurry of 3-20% (w/w) solid feed containing 30-80% (w/w) cellulose, with a cellulase enzyme complex. The solid cellulose-containing charge had a mean particle size ranging from 0.01 to 1 inch (0.0254-25.4 mm) in diameter. Perforated rotor blades were used for mixing. The teaching of the patent is to utilize a very high shear rate, on the order of 50,000 to 200,000 feet/minute/foot throughout the reaction zone.
US2002117167A describes enzymatic hydrolysis of hemicellulose in biomass material, comprising solubilizing at least a portion of hemicellulose and hydrolyzing the solubilized hemicellulose to produce at least one monosaccharide. The selected biomass is preferably an aqueous slurry of raw or pre-treated material. The biomass material may be any cellulosic material that includes hemicellulose. The process is described as being especially effective with grain fibers such as corn, wheat, rice, oats or barley. However, as noted in the examples, the cellulose is not hydrolyzed.
US2004005674A describes a process for enzymatic hydrolysis of lignocellulose. Degradation of lignocellulose to sugars comprises contacting the lignocellulose with at least one auxiliary enzyme and at least one cellulase. The lignocellulosic material was ground (the average fiber size of the material was not further specified) and had a low dry matter content (0.2 g of ground stover material in 10 ml of enzyme solution).
WO 2006/056838 describes a process for liquefaction and saccharification of polysaccharide-containing biomasses, having a relatively high dry matter content. It combines enzymatic hydrolysis with a type of mixing relying on the principle of gravity, ensuring that the biomasses are subjected to mechanical forces, primarily shear and tear forces.
The above processes are expensive to operate due to their high shear requirements and time to hydrolyze the feedstock. There exists therefore a need to use economical designs while at the same time treating high dry content biomass.